Decreasing body fat mass, which requires mobilizing and oxidizing adipose tissue triglyceride stores, is an important goal of therapy for obese persons. Exercise stimulates fat mobilization and oxidation and is an important component of obesity management. However, the effect of exercise on lipid kinetics in obese people is not well understood. This proposal is designed to : 1) evaluate whole body and regional lipolytic response to exercise in lean and obese women, 2) evaluate the relative contribution of plasma FFA and intramuscular triglycerides (IMTG) to total fat oxidation during exercise in lean and obese women 3)evaluate the effect of endurance exercise training on lipid kinetics and the relative contribution of plasma FFA and IMTG to whole body fat oxidation during exercise in lean and obese women. These studies will be performed in women with upper-body obesity (UBO;>0.85 waist:hip circumference ratio) because this group has been shown to have abnormalities in the regulation of lipolysis which may contribute to adverse health conditions such as diabetes, hyperlipidemia, and cardiovascular disease. Whole-body and regional (subcutaneous abdominal and femoral adipose tissue and IMTG ) lipid kinetics and oxidation will be measured during moderate intensity cycle ergometer exercise in lean (n=8) and UBO women (n=8) before and after 12 weeks of endurance exercise training without weight loss. Whole-body lipolysis and plasma FFA oxidation will be quantified using stable isotope methodology. Regional lipolytic activity will be estimated using micro dialysis probe methodology in conjunction with adipose tissue blood flow (Xe clearance) and muscle blood flow (ethanol dilution). Micro dialysis probes will be placed in subcutaneous adipose tissue of the abdomen and thigh, as well as within the vastus lateralis muscle. Since the increased heath risk in persons with UBO may be related to impaired regulation of lipid metabolism, a better understanding of whole-body and regional lipid mobilization and oxidation in persons with UBO and the effect of exercise on these responses may improve the therapeutic approach to these patients.